An automatic gain control circuit is widely used for an amplifier in an audio device such as a home audio system, car audio system, audio/video entertainment system, etc. to adjust the signal level associated with the amplifier. For example, a car audio system utilizes such an automatic gain control circuit with respect to a power amplifier to prevent overheating of a speaker as well as to avoid damages in the car audio system.
There are type of users who want to enjoy a very large volume of sound produced by such a car audio system when driving a car or otherwise using the car. Typically, such users specially mounts several loudspeakers in the car for large sounds including a subwoofer for bass sound. Power amplifiers for such loudspeakers and subwoofer are also installed in the car to produce large electric power so that very large sound of music, etc. can be produced by the car audio system.
A loudspeaker typically includes an acoustic transducer comprised of an electro-mechanical device which converts an electrical signal into acoustical energy in the form of sound waves. The acoustic transducer includes a voice coil which is a wire suspended between a pole piece and a permanent magnet and can oscillate in accordance with the reciprocal forces along the pole piece. The voice coil is attached to a cone shaped diaphragm which vibrates in response to the oscillation (reciprocal movement) of the voice coil. The vibration of the diaphragm produces acoustic energy in the air, i.e., a sound wave.
When an electrical signal is supplied to the voice coil, the electric current flowing therethrough generates heat because of resistance in the voice coil. Therefore, the temperature within the loudspeaker and its enclosure will increase. Additionally, when an amplifier for loudspeakers is driven at a range exceeding clipping points, an output of the amplifier is no longer linear or sinusoidal, which further heats up the amplifier and loudspeaker. Temperature rise in the voice coil increases the resistance of the voice coil, which results in a substantial portion of the input power of the loudspeaker to be converted to the heat, thereby lowering the efficiency and performance of the loudspeaker.
Such a loss of efficiency results in distortions of the reproduced sounds. When additional power is supplied to compensate for the increased resistance, additional heat is produced, again causes an increase in the resistance of the voice coil. At some point, any additional power input will be converted mostly into heat rather than acoustic output. Further, significant temperature rise can melt bonding materials in the voice coil or burn out the voice coil, resulting in permanent damage to the loudspeaker.
Therefore, it is necessary to limit the signal level from the amplifier that is input to the loudspeaker by means of the automatic gain control noted above. It is necessary that, when the user turns the volume of the amplifier extremely high and the temperature of the loudspeaker increases significantly, the automatic gain control dynamically adjusts the gain or signal level in the amplifier or output power level without shutting down the audio system so that the user can still enjoy the sounds. It is desired that the automatic gain control dynamically controls the signal level such as audio volume in a manner that is reliable as well as affordable. To protect the audio system from damage, it is also necessary to shutdown the system in a case where the user increases the sound volume too extreme for a long time that overheats the and may damage the audio system.